This invention relates generally to steelmaking processes and more particularly to a steelmaking process wherein an (electric) arc furnace with assisted combustion is used.
More specifically, this invention relates to a rapid steel melting process in an arc furnace for producing ordinary carbon steels and alloy steels from a cold charge of steel scrap as raw material and to related apparatus comprising an integrated combination of the arc furnace, special oxygen-fuel oil burners installed in the furnace for effecting assisted melting, and a fume evacuation system and incorporating a number of innovations in various parts of this apparatus.
By the provisions of this invention, the steelmaking apparatus can be operated continuously and efficiently over a long period with remarkably reduced time for maintenance shutdowns.
In recent years there has been widespread use of the so-called ultra-high-powered process (U.H.P. process) and, in some instances, auxiliary or assisted combustion systems, for the purpose of increasing the efficiency of (electric) arc furnaces for steelmaking in which a cold charge such as steel scrap is used as raw material.
In this U.H.P. process, which has in one bound become the focus of intense attention through the proposals and disclosures by W. E. Schwabe and others, use is made of a transformer of a capacity which is 1.5 to 2.0 times that of a conventional furnace of the same operational capacity, and the operation is conducted with a short arc. This process, however, is still accompanied by several problems, among which are high investment cost or initial cost for equipment and the limitation of location to one where a large power supply is available. Furthermore, in actual operation, the following difficulties are encountered because the operation is carried out with a low power factor of low voltage and high current.
1. In the U.H.P. process, since the operation is generally carried out with high current, much Joule's heat and electromagnetic force are generated and accelerate damage to the electrode holders and oxidation and wear of the electrodes. Furthermore, thermal stresses due to temperature difference between the exterior and interior of the electrode tend to produce damage such as breaking, splitting, and spalling of the electrodes.
2. In general, as the electrical capacity is increased, the melting time for steel scrap is reduced, and the ratio of the total power-on time A from start to tap (i.e., the time during which steelmaking work is actually carried out) and the power-off time B from tap to start (i.e., the time from tap to the succeeding power-on and the time such as that spend in charging of scrap steel and repair of parts such as furnace wall refractories), that is, the effective operational rate or practice ratio A/(A+B) .times. 100% of the furnace, tends to become low, and the effectiveness of equipment investment decrease in some cases.
3. Since the distance between the steel scrap and the electrodes fluctuates, in the melting period wherein the bulk density of the steel scrap in the furnace is low, operation with a short arc is more disadvantageous than that with a long arc in order to shorten the melting time.
4. Concentrated local damage due to melting of the refractories of the furnace walls and roof caused by powerful electric arcs generated between the electrodes and the steel scrap is severe.
Because of the above enumerated difficulties, the practice ratio of the furnace as a whole is lowered, and, when considered from the viewpoint of long-period and continuous operation, the process entails several features which are not advantageous improving productivity and economy.
On one hand, the so-called "Shell Toroidal" burner used in the Fuel, Oxygen, and Scrap (F.O.S.) process developed in England is at present a representative example of an auxiliary burner for an assisted melting process. The advantages afforded by the installation of this system in an arc furnace for steel-making are as follows.
1. This system can be installed with relative ease in an already existing arc furnace.
2. A tremendous equipment investment is not required as in the U.H.P. process.
In actual practice, however, several problems are encountered, the principal being as follows, whereby there is a limit to the effectiveness of the system.
An ordinary burner of the type representably by the "Shell Toroidal" burner is used for the purpose of preheating and melting steel scrap by means of a high-temperature flame. In a closed furnace such as an arc furnace, however, there is no combustion chamber, and it is difficult to use a large quantity of fuel. Furthermore, as a natural consequence, it is necessary to generate a flame of short frame. Provisions have been devised for this necessity but give rise to the following problems.
1. Low thermal efficiency
In the case of a "Shell Toroidal" burner in which the fuel is atomized with pure oxygen, the theoretical combustion temperature becomes approximately 2,800.degree. C. On one hand, in a high-temperature region, the dissociation coefficients of CO.sub.2 and H.sub.2 O increase, and the latent heats become even higher than 50 percent.
The average temperature of the steel scrap is very low, of course, and when the combustion gases contact this material to be heated, combustion takes place on the outer surface, where by recombination occurs. Consequently, heat of reaction is generated. By advantageously utilizing this characteristic, rapid heating becomes possible. However, if an error is made in the use of the burner, the latent heat in the exhaust gases will increase and give rise to a great drop in thermal efficiency. It can be seen from this that during the initial period when the cold charge has been placed in the furnace, the process is relatively effective, but the thermal efficiency decreases with increase in the temperature of the material being heated.
2. Limit to period of use
While the steel scrap oxists with an appropriate bulk density in the furnace, the flame issuing from the burner disperses suitably within the body of steel scrap pieces, but as the quantity of molten steel increases, and the material being heated becomes dense, its surface area becomes small. For this reason, the coefficient of heat absorption decreases, and, at the same time, the temperature of the exhaust gas increases. As a consequence, in general, effectiveness is afforded in only the first part, excluding the last period of the melting.
3. Much damage to the furnace facilities
Severe local melting damage occurs at each burner orifice and frame contact parts, and, at the same time, excessive rise in the temperature of the atmosphere within the furnace generally hastens melting damage of the furnace wall bricks, increases the brick consumption, and entails economic disadvantage. On one hand, the rise in the waste gas temperature is accompanied by a rise in the temperature of the cooling water of the furnace body, and problems associated with the cooling water piping readily arise. Furthermore, in the dust collector, also, trouble such as breakage of the bag filter and deficient suction performance occur and give rise to an increase in the damage to the accessory facilities thereof.
4. Difficulty in maintenance because of complexity of the mechanism of the burner and entire apparatus.
In the case of a burner in which the fuel is caused to undergo combustion by atomizing it directly with pure oxygen, damage to its working end is caused by back firing of the flame at a high temperature. Furthermore, because of special provisions such as a safety device for holding any back fire within the burner cylinder, a complicated structure at the burner tip for causing a toroidal curve to be defined, and a ratio setter for proportional control of the fuel-air ratio, the entire burner device becomes complicated and expensive in production and difficult to maintain.
Because of the above enumerated problems 1 and 2, the limit to the output produced in this process is generally considered to be of the order of 20 percent of the total input energy of the arc furnace from the operational and economic viewpoint, although this depends on the factors of the efficiency of the burners and the prices of electric power, oxygen, and fuel oil. By the practice of this invention, the output is increased by the unique mechanism of melt cutting and melting of the cold charge of scrap and the like by the burners. Furthermore, because of the above problems 3 and 4, difficulties in maintenance are encountered, but by the improvements in construction of the burners for injection of oxygen and fuel oil and in the burner mounting parts and other novel innovations according to the invention, these difficulties are overcome and remarkable improvements are attained as described below.